1. Field of the Invention
The present invention concerns a standing wave barrier to suppress standing waves on a cable, the standing wave barrier being of the type having an opening that is fashioned to accommodate the cable. Moreover, the invention concerns a magnetic resonance tomography device with a transmission antenna and a number of such standing wave barriers arranged within the transmission antenna.
2. Description of the Prior Art
Magnetic resonance tomography is a technique in widespread use for the acquisition of images of the inside of the body of a living examination subject. In order to acquire an image with this method (i.e. to generate a magnetic resonance exposure of an examination subject), the body or a body part of the patient that is to be examined must initially be exposed to an optimally homogeneous, static basic magnetic field (most often designated as a B0 field) which is generated by a basic field magnet of the magnetic resonance tomography device. The basic field magnet is essentially shaped like a cylinder and is relatively long in order to achieve within it an optimally long region in which the homogeneous static basic magnetic field exists. Typically there is enough space in the cylindrical structure that a patient to be completely accommodated therein.
Rapidly switched gradient fields for spatial coding. that are generated by gradient coils, are superimposed on this basic magnetic field during the acquisition of the magnetic resonance images. Moreover, RF pulses of a defined field strength are radiated with a radio-frequency antenna into the examination volume in which the examination subject is located. This radio-frequency antenna permanently installed in the apparatus is often designated as a “transmission antenna”, and additional common designations are, for example, body resonator, body coil or whole body antenna. The antenna is frequently designed in a form known as a birdcage antenna (the design of which is explained in more detail below) or as a saddle coil. The transmission antenna is localized within the basic field magnet and typically have a length between 30 cm and 60 cm.
The nuclear spins of the atoms in the examination subject are excited by the RF pulses so that they are deflected by an amount known as an “excitation flip angle” out of their equilibrium state, which runs parallel to the basic magnetic field B0. The nuclear spins then precess around the direction of the basic magnetic field B0. The magnetic resonance signals that are thereby generated are received by the radio-frequency reception antennas. The reception antennas can either be the same antennas with which the radio-frequency pulses were radiated, or separate reception antennas. The magnetic resonance images of the examination subject are ultimately generated on the basis of the received magnetic resonance signals. Every pixel in the magnetic resonance image is associated with a small body volume (known as a “voxel”), and every brightness or intensity value of the pixels is lined with the signal amplitude of the magnetic resonance signal that is received from this voxel.
In order to obtain an optimally good signal-to-noise ratio, the reception antennas are often formed as local coils that are separate from the transmission antennas. These local coils are optimized in terms of their geometry and their reception profile for different body regions, and are positioned optimally close to the body of the examination subject (patient). Shielded coaxial cables are typically used for the conducting the magnetic resonance signal from the local coil to a signal processing system.
In conventional magnetic resonance tomography devices, the local coil is connected with a first coaxial cable that is plugged in at a patient table. Connected with the socket of the patient table is an additional coaxial cable that conducts the magnetic resonance signal from the patient table and relays it to the signal processing system. Radio-frequency currents on the conductor shield (outside conductor) of the coaxial cable are induced due to the electrical and magnetic fields that occur during the transmission phase of the radio-frequency pulses. These currents are known as waves on a cable and, without suitable suppressing measures, can lead to image interference and—in the worst case—even to an endangerment of the patient.
For completeness, it is noted that the local coil itself can also be used as a transmission antenna in special situations. Standing waves that must then be suppressed with a standing wave barrier can arise in this case. For example, such a situation is present with head coils that are used as transmission antenna coils. However, in such a head coil the cable does not run within transmission coil (as in the permanently installed transmission coil) but rather runs past the head coil (external to the head coil), and standing waves are likewise generated in the outer conductor of the cable.
Standing wave barriers integrated into the patient bed, which represent a high-ohmic impedance Zo for the radio-frequency currents, are used to suppress the standing waves. For example, the impedance Zo can be realized by a parallel resonator. The coaxial cable is thereby wound on a coil and is connected at the ends of the winding of the conductor shield that is generated in this way with a capacitor connected in parallel with said winding. Such standing wave barriers are necessary in all conductors that lead through the transmission antenna. Such a known standing wave barrier 1 is depicted in FIG. 5. A coaxial cable 2 with a conductor shield 4 that is open to the outside and that surrounds a plurality of inner conductors 3 is would on a coil 26. The coil 26 is connected at both of its ends with terminals of a first capacitor 14 with whose help the resonance of the parallel oscillating circuit that is formed in this way can be adjusted. The inner conductors 3 are insulated in a conventional manner from the conductor shield 4.
The impedance Zo can also be realized by a λ/4 wave trap (see FIG. 6). In such a solution, a copper tube 27 with a length L is slid over the outer conductor 4 of the cable 2. One end—at the one opening—of the copper tube 27 is directly soldered with the outer conductor (see first solder point 28) and the other end—at the other opening—of the copper tube 27 is electrically connected with the outer conductor 4 via what are known as shortening capacitors (see first capacitor 14) that on the one end are soldered with the external conductor 4 and on the other end are soldered with the copper tube 27 (see second solder point 29). In many cases, multiple signal-conducting inner conductors 3 are also merged and surrounded with a single outer conductor 4. The expression “shortening capacitor” actually originates from radio engineering and there designates a capacitor that serves to electrically shorten antennas. There it is connected in series with the antenna and should be of optimally high quality.
The radio-frequency current (RF current) I that forms the standing wave to be suppressed and flows in the outer conductor 4 is also visualized in FIGS. 5 and 6. Both embodiments of the standing wave barrier according to the prior art that are described in the preceding are characterized by, during operation of the magnetic resonance tomography device, the standing wave barriers mounted at or in the patient bed being moved through the transmission antenna together with the patient and the local coils.
In the known solution to suppress the standing waves, a significant number of standing wave barriers at relatively short intervals relative to one another is consequently necessary in order to ensure the desired suppression of the standing waves along the entire length of the patient bed that is usable within the transmission antenna. This solution can be relatively complicated, and therefore also expensive. Furthermore, an optimally flexible use of the patient bed is not ensured because the integrated standing wave barriers can only be tuned to a resonance frequency. Therefore, a patient bed that is adapted to a resonance tomography device operated with 1.5 Tesla (magnetic resonance frequency for H+ nuclei=62.66 MHz) cannot be used in a magnetic resonance tomography device operating with 3 Tesla, for example (magnetic resonance frequency for H+ nuclei=125.32 MHz), but such alternative use would be desirable. An additional disadvantage is the fact that the outer conductor 4 is a fixed and precisely localized component of the standing wave barrier.
It is noted that different realizations for standing wave barriers are known in other technical fields of engineering. For example, isolating transformers, capacitive couplers as well as ferrite core chokes are known, by every solution entails inherent advantages and disadvantages. For example, ferrite core-based solutions in which a cable is directed through an opening in the ferrite core so that the ferrite core completely surrounds the cable, cannot be used in magnetic resonance tomography because the ferrite cores would be driven to saturation due to the relatively high magnetic field.